LED with high thermal conductivity particles in phosphor conversion layer

ABSTRACT

In one embodiment, a solid cylindrical tablet is pre-formed for a reflective cup containing an LED die, such as a blue LED die. The tablet comprises uniformly-mixed phosphor particles and transparent/translucent particles of a high TC material, such as quartz, in a hardened silicone binder, where the index of refraction of the high TC material is matched to that of the silicone to minimize internal reflection. Tablets can be made virtually identical in composition and size. The bulk of the tablet will be the high TC material. After the tablet is placed in the cup, the LED module is heated, preferably in a vacuum, to melt the silicone so that the mixture flows around the LED die and fills the voids to encapsulate the LED die. The silicone is then cooled to harden.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.14/782,774, filed Oct. 6, 2015 which is a National Stage Entry ofPCT/IB14/60366, filed Apr. 2, 2014 which claims priority fromProvisional Application No. 61/809,451, filed Apr. 8, 2013, which areincorporated by reference as if fully set forth.

FIELD OF INVENTION

This invention relates to light emitting diodes (LED) and, inparticular, to an LED die in a reflective cup filled with a wavelengthconversion layer.

BACKGROUND

It is well known to mount a blue LED die in a conical reflective cup andthen fill the cup with a drop of a viscous phosphor mixture. The mixtureis then cured. If the phosphor is a YAG phosphor, white light will becreated by the combination of the yellow-green YAG emission and bluelight leaking through the cured phosphor mixture.

A common phosphor mixture comprises phosphor powder in a transparentbinder, such as silicone or epoxy. Such binders have very poor thermalconductivity (TC), such as 0.1-0.2 W/mK, and make up the bulk of thephosphor mixture. Accordingly, the overall TC of the hardened phosphormixture is very low.

Due to the low TC, heat is not conducted from the LED die through thephosphor mixture and to the cup walls and base. Accordingly, the LEDoperates at a relatively high temperature. If the LED operates at a hightemperature, its light output becomes lower and its color shifts overits lifetime. Therefore, the low TC phosphor mixture limits suchreflector cup applications to low and medium current/heat LEDs.

What is needed is a technique that allows a high brightness/highcurrent/high heat LED to be encapsulated by a phosphor mixture in areflective cup.

In addition to the problem addressed above, the phosphor mixture musthave a low viscosity in order for it to be used with a small-diameterdispensing nozzle under practical injection pressures and to fill in thevoids around the LED die. A typical phosphor mixture may produce unevenphosphor density in the deposited mixture due to the rapid slurry flowand the low viscosity of the mixture. Additionally, the phosphor mixtureviscosity permits phosphor particles in the dispensed volume to settleto the bottom of the cup before the phosphor mixture is cured. Due tothis phosphor settling inside the cup, it is difficult to maintain thetargeted color of the package. Further, it is difficult to repeatedlydispense identical amounts of the viscous phosphor mixture in the cups,which causes non-uniformity of color from LED module to LED module.

What is needed is a technique to improve the color consistency from LEDmodule to LED module and to improve the color uniformity of the package.

SUMMARY

The present invention solves the problems addressed above.

In one embodiment, a solid cylindrical tablet is pre-formed for eachreflective cup containing an LED die, such as a blue LED die. The tabletdiameter is preferably such that the side edges of the tablet abut thecup walls while the bottom of the tablet contacts the top of the LEDdie. This causes the tablet to be centered. However, precise placementis not required to achieve the goals of the invention.

The tablet has an overall high thermal conductivity (TC) and comprisesuniformly-mixed phosphor particles and transparent (or translucent)particles of a high TC material in a silicone binder, with the index ofrefraction of the high TC material matched to that of the silicone. Theparticles are typically all on the order of a few microns in diameter,but a wide range of sizes is suitable for the invention.

The tablets may be made by mixing the solid phosphor and high TCparticles in a viscous silicone binder at an elevated temperature (abovethe melting point of the silicone) to form a slurry, and the slurry ispoured in a mold to make a uniformly thick sheet. The slurry is thencooled to harden the sheet prior to any settling of the particles. Thetablets are then stamped or “punched” out so they are virtuallyidentical in composition and size.

In another embodiment, the silicone is viscous at room temperature andcured by heat, UV, or other methods.

The cured tablets or sheet can also be tested or further processed(e.g., thinned) so as to be precisely matched to a target color point.Thus, all completed LED packages/modules will generate a consistenttarget color.

In one embodiment, the high TC particles are a quartz, such as themineral crystobalite. Crystobalite has a thermal conductivity of morethan 3 W/mK, which is at least an order of magnitude greater than thatof silicone. Glass beads/particles or other suitable materials, withtheir index of refraction matched to that of the silicone, may also beused. The high TC particles should be transparent or translucent. Thebulk of the phosphor mixture will be the high TC material.

The high TC particles and the silicone are selected so that theirindices of refraction (n) are as close as practical to one another tominimize internal reflection. The index of refraction of both materialswill typically be around 1.49. Silicone and quartz powders with avariety of indices of refraction are commercially available.

After the tablet is placed in the cup, the LED module is heated,preferably in a vacuum, to melt the silicone so that the mixture flowsaround the LED die and fills the voids to encapsulate the LED die. Themelting temperature of the silicone is higher than the maximum operatingtemperature of the LED module. The mixture conforms to the cup shape andwill have a substantially flat top surface. The viscosity of siliconecan be made very low when heated. After melting, the mixture will have asubstantially uniform distribution of phosphor and a substantially flattop surface.

The mixture is then cooled or otherwise cured to harden.

By using solid pre-formed tablets, handling of the phosphor mixture ismuch easier than handling of the prior art viscous mixture, and thevolume of the phosphor mixture per cup is more precise.

Since the phosphor mixture has a high TC, higher brightness/higher heatLED dies may be used since the heat is conducted away by the phosphormixture. Further, the phosphor mixture can be made to have a veryuniform density of phosphor for high color uniformity. Further, sinceeach tablet may be easily formed to have the same characteristics, therewill be good color consistency from LED module to LED module. Further,since the phosphor mixture is not dispensed as a liquid, the top surfaceof the phosphor mixture may be very flat so there is good color vs.viewing angle uniformity. Other advantages exist.

Balls or other shapes may be used instead of the tablets. In someembodiments, a tablet sheet is pressed and thinned to a precisethickness, then the tablets are stamped out and placed in the cup. Eachtablet is then melted to conform to the cup and the LED die.

The process may be formed on a wafer scale, where many LED modules aresimultaneously formed.

Quantum dots may be used instead of phosphor.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a magnified cross-sectional view of a single pre-formed tabletfor wavelength-converting light emitted by an LED die in a reflectivecup.

FIG. 2 is a cross-sectional view of a plurality of conical reflectivecups supported on a substrate having electrodes for attachment to LEDdie electrodes.

FIG. 3 illustrates flip chip LED dies mounted in the cups.

FIG. 4 illustrates the tablets positioned in each of the cups.

FIG. 5 illustrates the LED modules after undergoing a heating step undera vacuum to cause the silicone binder to melt and the phosphor mixtureto encapsulate the LED dies.

FIGS. 6-9 illustrate the same process as FIGS. 2-5 but performed on LEDmodules having multiple LED dies in each reflective cup.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 is a magnified cross-sectional view of a single pre-formed tablet10 for wavelength-converting light emitted by an LED die in a reflectivecup. In one embodiment, the tablet 10 is cylindrical and has a diameterof about 2-4 mm, depending on the size of the cup. The height of thetablet 10 will typically be about 2-3 mm, depending on the volume of thecup.

The tablet 10 diameter is preferably such that the side edges of thetablet 10 abut the cup walls while the bottom of the tablet 10 contactsthe top of the LED die. This causes the tablet 10 to be centered in thecup. However, precise placement is not required to achieve the goals ofthe invention.

Instead of a cylindrical form, a rounded form or even a ball may be usedfor each cup.

The tablet 10 comprises uniformly-mixed phosphor particles 12A, 12B andhigh TC particles 14 in a silicone binder 16. Phosphor particles 12A maybe YAG phosphor and are shown as white large circles. Phosphor particles12B may be red light emitting phosphor and are shown as shaded largecircles. High TC particles 12 are shown as small white circles.

The phosphor particles may be all of one type (e.g., a YAG phosphor 12A)or of different types (e.g., a combination of YAG 12A and a red lightemitting phosphor 12B). Although the phosphor particles are shown asuniformly large and the TC particles as uniformly small, any combinationof large and small non-uniform phosphor particles, large and smallnon-uniform TC particles are contemplated and are included within thescope of the invention. Likewise the exemplary YAG phosphor particles12A may be larger or smaller than the exemplary red phosphor particles12B. Typically the particles 12A, 12B and 14 are all on the order of afew microns in diameter, but a wide range of sizes is suitable for theinvention.

The tablets 10 may be made by mixing the solid particles 12A, 12B, and14 in the desired ratio in a viscous silicone binder at an elevatedtemperature. The viscosity of the silicone reduces with temperature and,ultimately, the particles 12A, 12B, and 14 will be uniformly distributedthroughout the binder. The resulting mixture (a slurry) is poured in amold to make a uniformly thick sheet. The mixture is then hardened bycooling or other curing technique. The tablets 10 are then stamped out,sawed, or separated in other ways, so they are virtually identical incomposition and size.

In another embodiment, the silicone is viscous at room temperature andcured by heat, UV, or other methods.

The sheet/tablet may be tested after being formed and categorized toprecisely match the tablets 10 to a particular LED die peak wavelength.

In one embodiment, the high TC particles 14 are a quartz, such as themineral crystobalite. The high TC particles 14 may be formed by grindingand grading the resulting powder. Crystobalite has a thermalconductivity of more than 3 W/mK, which is at least an order ofmagnitude greater than that of silicone (e.g., 0.2 W/mK). Glassbeads/particles, a crystalline silica, or other suitable high TCparticles, index matched to the silicone may also be used. The high TCparticles 14 should be transparent or translucent.

The high TC particles 14 and the silicone 16 are selected so that theirindices of refraction (n) are as close as practical to one another tominimize internal reflection. The index of refraction of both materialswill typically be around 1.49. Silicone and quartz powders with avariety of indices of refraction are commercially available. The indicesof refraction should be preferably matched to within about 0.1.

The phosphor particles 12A and 12B may comprise a mixture of red andgreen phosphor particles, or YAG phosphor particles, or any other typesof phosphor(s), depending on the desired color emission of the LEDmodule.

The bulk of the phosphor mixture forming the tablet 10 will be the highTC material, so the resulting phosphor mixture, when encapsulating theLED die, will be a good conductor of heat, allowing the LED die to be ahigh brightness/high current/high heat type. FIG. 2 is a cross-sectionalview of a plurality of conical reflective cups 20 supported on asubstrate 22 having electrodes 24 for attachment to LED die electrodes.The electrodes 24 may extend to pads on the bottom of the substrate 22for soldering to a printed circuit board after the substrate 22 issingulated. The substrate 22 will typically contain a large array ofsuch cups 20 so that the processing is on a wafer scale to reduce costsand handling.

The cups 20 may be reflective metal rings, or may be resin coated with areflective metal film or painted, or may be any other reflectivematerial. The height of the cup 20 is typically on the order of about1.5-3 mm for a single LED die.

FIG. 3 illustrates conventional flip chip LED dies 26 mounted in thecups 20 and connected to the substrate electrodes 24. In one embodimentthe LED dies 26 emit blue light.

FIG. 4 illustrates the tablets 10 positioned in each of the cups 20 by aconventional pick and place machine. Note how the tablets 10 arecentered due to the edges of the tablets 10 contacting the cup 22 walls.The tablets 10 may or may not extend above the top of the cups 20,depending on the height of the cups 20.

In FIG. 5, the substrate 22 is placed in a vacuum and heated to raisethe temperature of the silicone 16 above its melting point. Due togravity, surface tension, and the vacuum, the void will be filled by themolten phosphor mixture to encapsulate the LED die 26. The structure isthen cooled to room temperature to harden the silicone 16, or thesilicone 16 is cured using other techniques (e.g., UV light). If the LEDdies emitted UV light, and the binder was the type to be cured with UV,the LEDs may be energized to cure the binder.

The resulting phosphor mixture 30 has a substantially flat top surface,a precise volume of the phosphor mixture 30 is provided, and the densityof the phosphor particles 12A and 12B (FIG. 1) is substantially uniform.Accordingly, the light emitted by each LED module 32 will be the same,and the color emission will be substantially uniform over a wide rangeof viewing angles. The relatively high viscosity of the phosphormixture, the small sizes of the phosphor particles, the relatively shorttime that the phosphor mixture is in a melted state, and the slowmovement of the phosphor mixture ensures that the phosphor particlesremain uniformly distributed and do not settle.

The substrate 22 is then singulated, such as by sawing, to separate theLED modules 32.

FIGS. 6-9 illustrate the same process as FIGS. 2-5 but performed on LEDmodules having multiple LED dies 26 in each reflective cup.

In FIG. 6, relatively large reflective cups 40 are provided on asubstrate 42, where the size depends on how many LED dies are to bemounted in each cup 40.

In FIG. 7, the LED dies 26 are mounted in the cups 40. In the example ofFIG. 7, a circular array of 16 LED dies 26 connected in series aremounted in each cup 26. FIG. 7 is a cross-section across the center lineof the cups 40. Although a circular array of LED dies is described, anyother suitable arrangement such as a rectangular array of dies iscontemplated and included within the scope of the invention.

In FIG. 8, a large tablet 46, which may be identical to that of tablet10 in FIG. 1 but larger, is positioned in each cup 40.

In FIG. 9, the substrate 42 is heated in a vacuum to melt the siliconeto encapsulate the LED dies 26, as previously described.

The substrate 42 is then singulated to form individual LED modules 48.

In the various embodiments, the LED die 26 may be a blue die and thephosphor creates any desired color emission such as a white lightemission for general illumination. Due to the high TC of the phosphormixture, the LED dies 26 may be high brightness types, allowing the LEDmodules to be used for general illumination applications.

The phosphor particles 12A and 12B may be replaced by quantum dots orother wavelength converting particles. The silicone 16 may be replacedwith any other suitable binder that can be softened or liquefied afterbeing deposited in the cup and then hardened to encapsulate the LED die.LED dies other than flip chips may be used. In the above example, thehigh TC particles 14 had a thermal conductivity greater than ten timesthat of the binder (silicone 16); however, even a thermal conductivitygreater than three times that of the binder will have beneficialeffects. In the context of the present invention, the high TC particlesare suitable materials whose TC is at least three times greater than thebinder material to substantially increase the TC of the wavelengthconversion mixture.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A method of fabricating a light emitting diode(LED) module comprising: providing a reflective cup containing at leastone LED die; positioning a solid piece on the at least one LED die andwithin the reflective cup, the solid piece abutting inner walls of thereflective cup to create a void below the solid piece, the solid piececomprising: a binder; high thermal conductivity particles, an index ofrefraction of the binder and an index of refraction of the high thermalconductivity particles being substantially equal, and a thermalconductivity of the high thermal conductivity particles being greaterthan a thermal conductivity of the binder; and wavelength conversionparticles that convert a wavelength of light emitted by the at least oneLED die to a different wavelength, the wavelength conversion particlesand the high thermal conductivity particles being uniformly mixed in thebinder; softening the positioned solid piece to conform to a volumedefined by the inner walls of the reflective cup and to encapsulate theat least one LED die to draw heat away from the at least one LED die;and hardening the softened solid piece after encapsulation of the atleast one LED die.
 2. The method of claim 1, the step of softening thepositioned solid piece further comprising: heating the positioned solidpiece to melt the binder.
 3. The method of claim 2, the step ofsoftening the positioned solid piece being performed in a vacuum.
 4. Themethod of claim 1, the binder comprising silicone.
 5. The method ofclaim 1, the wavelength conversion particles comprising at least onephosphor.
 6. The method of claim 1, the high thermal conductivityparticles comprising at least one of a quartz, crystalline silica,crystobalite, and glass.
 7. The method of claim 1, the high thermalconductivity particles comprising a majority of the solid piece.
 8. Themethod of claim 1, the positioning the solid piece in the reflective cupfurther comprising: positioning the solid piece to be substantiallycentered in the reflective cup.
 9. The method of claim 1, the reflectivecup being conical.
 10. The method of claim 1, further comprising:forming the solid piece by: mixing the wavelength conversion particlesand the high thermal conductivity particles in the binder while thebinder is softened to form a slurry; forming a sheet of the slurry;hardening the binder; and separating the resulting hardened sheet intosubstantially identical solid pieces.
 11. The method of claim 10, theforming the solid piece further comprising: thinning at least one of thesubstantially identical solid pieces to match a target color point. 12.The method of claim 1, the solid piece having a generally cylindricalshape.
 13. The method of claim 1, the providing the reflective cupcontaining at least one LED die further comprising: providing aplurality of identical reflective cups on a substrate, each reflectivecup containing at least one LED die.
 14. The method of claim 1, thehardened piece after encapsulation conducting heat from the at least oneLED die to the reflective cup and to a base of the reflective cup. 15.The method of claim 1, the thermal conductivity of the high thermalconductivity particles being three times the thermal conductivity of thebinder.
 16. The method of claim 1, the hardening the softened solidpiece after encapsulation of the LED die causes the hardened solid pieceto have a substantially flat top surface across the reflective cup.